The present invention relates to optical resonators. More particularly, it relates to optical resonators using ring resonators which may be temporarily tuned by the application of laser energy to resonate at a selected frequency. The invention also relates to methods of tuning the optical resonators to adjust the frequency at which resonance occurs.
As technology advances, the volume of data around the world is growing at an exponential rate. Everything from increased usage of the global information networks (e.g., the Internet), video conferences, and mobile phones relies on efficient data transfer. Reductions in the distance that electrons need to travel within and between components have provided the dramatic increases in device speeds. Increases in the speed of electronic devices through the 1980s and 1990s largely resulted from reductions in size of microelectronic components. However, microelectronics communications networks have physical limitations that effectively limit the volume of data that can be transferred. As devices encroach upon the physical limits of component density and complexity, device reliability and speed advances for new devices are declining.
Optical communication of data (e.g., sending photons through optical fiber, rather than sending electrons through wire) is already widely implemented for certain connections and communications. Optical connections, optical switching, and all-optical circuits provide ways for photons to travel and be routed, in place of electrons traveling through electronic circuits. Wavelength division multiplexing (WDM) provides a way to send even more data through optical components (such as fiber) by mixing light of different frequencies in the same fiber. A demultiplexer separates a specific frequency of light from a fiber.
Photonic devices and microphotonics provide significant potential for furthering the advancement of technology devices historically served by microelectronics because they also enable large volumes of data to travel along optical fibers and be routed to their final destinations. A primary reason that all-optical circuits have not yet been implemented is that there are manufacturing problems related to photonic device fabrication, such as meeting index of refraction specifications related to making these photonic devices. The small feature size required for photonic devices, as well as small tolerances for physical specifications of photonic devices, have delayed the discovery and use of mass manufacturing techniques for these devices.
A ring resonator is a device that is designed to resonate at a specific frequency of light in order to enable the ring resonator to selectively couple light at the resonant frequency from an input waveguide to an output waveguide. Typically, light energy is coupled into the ring resonator via evanescent coupling. In this coupling mode, the ring resonator is placed in close proximity to but not touching either the input waveguide or the output waveguide. Another common configuration, however, is to have the ring resonator touching one or both of the input and output waveguides in which case, light is transferred via leaky-mode coupling. The light in the input waveguide that is not in resonance passes by the ring resonator with only a small transmission loss. Using this technique, a ring resonator functions as a passive router, routing an individual frequency of light (i.e. the resonant frequency) from the input waveguide onto a separate output waveguide. When multiple ring resonators are used, they function as multiple passive routers, routing a plurality of individual frequencies of a WDM light signal from an input waveguide onto separate output waveguides at the same individual frequencies.
Thus, ring resonator devices have two important attributes for optical communications, functionality and compactness. Functionality refers to the fact that a wide range of desirable filter characteristics can be synthesized by coupling multiple resonators. Compactness refers to the fact that ring resonators with radii of less than 25 xcexcm can lead to large scale integration of devices with densities on the order of 104 to 105 devices per square centimeter.
It is desirable for ring resonators to be precisely tuned to achieve good discrimination between frequencies. A ring resonator may be designed to be in resonance at a specific frequency in order to enable the ring resonator to couple light signals at that frequency from an input waveguide to an output waveguide. Using a demultiplexer with ring resonators, the light of various frequencies may be separated when the light is incident upon the demultiplexer. A series of ring resonators may be used to separate the light at discrete frequencies, and route the discrete frequencies to specific devices for data handling and processing.
Ring resonators may be fabricated using known methods such as x-ray or optical lithography. X-ray and optical lithography allow manufacturers to create the very small feature sizes used to implant ring resonators. After ring resonators are manufactured, it may be desirable to tune the resonator to adjust its resonant frequency. Temperature control has been used as one possible tuning method. However, temperature change may not be localized enough to independently tune closely spaced devices. Ultraviolet light induced refractive index changes in a spin coated photosensitive polymer has also been used to tune resonators.
One embodiment of the invention is a tuned optical resonator comprising a substrate, an input waveguide formed on the substrate and an optical resonator waveguide formed on the substrate. The optical resonator waveguide has a resonant frequency, exhibits a first index of refraction, temporarily exhibits a second index of refraction responsive to application of laser energy and temporarily resonates at the resonant frequency at which the laser energy is being applied.
Another embodiment of the invention is a method of temporarily tuning an optical resonator having a substrate, an input waveguide, and an optical resonator waveguide exhibiting an inherent index of refraction coupled to the input waveguide. The method comprises measuring a resonance characteristic of the optical resonator, applying a first amount of laser energy to the optical waveguide to permanently change the inherent index of refraction to a first index of refraction and temporarily illuminating the resonator with a second amount of laser energy to change the first index of refraction to a second index of refraction while the optical resonator is being illuminated in order to temporarily tune the optical resonator to a desired resonance characteristic.
Another embodiment of the invention is an optical demultiplexer comprising a substrate, a first optical waveguide formed on the substrate, which guides a multiplexed light beam having at least one frequency of light. An optical device configured to receive the light beam from the first optical waveguide and temporarily tuned to receive and resonate at the at least one frequency of light and to temporarily route the at least one frequency of light. The optical device comprises an optical resonator formed on the substrate. The optical resonator is temporarily modified by application of laser energy to tune the resonant frequency of the optical resonator to the at least one frequency of light. The optical device also comprises a second optical waveguide formed on the substrate and configured to temporarily receive light at the at least one frequency of light from the optical resonator.
Yet another embodiment is an optical demultiplexer comprising an input optical waveguide carrying a plurality of frequencies of light, a plurality of optical devices configured to receive the light beam from the input optical waveguide. Each of the optical devices temporarily resonates at least one of the plurality of frequencies of light. There are also a plurality of output optical waveguides optically coupled to respective optical devices for temporarily receiving light at respective ones of the resonant frequencies.